Nonlinear systems can be used to detect small signals in the presence of signals with significantly larger amplitudes. Such detection typically requires a high spur-free dynamic range (SFDR), which is a measure of the amplitude of the fundamental with respect to the amplitude of the largest harmonic tone or spur. These nonlinear systems can include several nonlinear functions that interact with one another to provide the desired output. One example is a radio frequency (RF) system with an RF receiver, an analog to digital converter (ADC) driver, an anti-alias filter and an ADC. The RF system receives a signal and converts it into the digital domain with the ADC. An anti-alias filter is required to remove high frequency tones that can alias into the baseband frequency range when subsequently sampled by the ADC.
ADC designers and manufacturers often focus on improving ADC linearity, without regard to the nonlinear distortion created by the other functions in the RF system signal chain, for example the RF receiver, ADC driver and anti-alias filter. Achieving a desired SFDR for a nonlinear system is best achieved when each function of the system is properly balanced with respect to SFDR because the system is only as strong as the weakest link. For example, with reference to FIG. 1A, a 16-bit ADC shown as element 18 with an SFDR of 90 dB requires the output of the anti-alias filter 16 to have an SFDR of at least 90 dB. Similarly, the output of the ADC driver 14 and the output of the RF receiver 12 each require an SFDR of at least 90 dB.
One way of achieving high SFDR for an RF system, shown in FIG. 1A, is to use an ADC driver 14 with very low distortion and an anti-alias filter using passive components such as inductors (L) and capacitors (C), also referred to as an LC filter. The low distortion ADC driver 14 dissipates a lot of power on the order of 1-2 Watts. The anti-alias LC filter is typically very large physically. Both the high power consumption and the large filter size make this approach undesirable.
An additional source of high power consumption arises from the need to impedance match the separate components in FIG. 1A with a fifty-ohm reference. Specifically, the RF receiver 12 and the ADC driver 14 are matched with a fifty ohm reference 20, the anti-alias filter 16 and the ADC driver 14 are matched with a fifty ohm reference 22 and the ADC 18 and the anti-alias filter 16 are matched with a fifty ohm reference 24. A fifty-ohm reference is typically needed or any interconnect that has an electrical length longer than approximately one tenth of the wavelength of the highest frequency tone in a Fourier transform of the propagated signal. This fact motivates integrating the various components onto a monolithic semiconductor chip.
A second way of achieving high SFDR for an RF system, is to use a surface acoustic waver (SAW) filter 34 shown in FIG. 1B in place of the LC filter 16 shown in FIG. 1A. A SAW filter consumes less physical space than an LC filter, however the SAW filter suffers from significant pass-band signal loss. For example, a SAW filter with a 40 MHz bandwidth and a 90 dB stop-band rejection may attenuate the pass-band signal by more than 20 dB. This attenuation, requires the ADC driver 38 in FIG. 1B to have higher gain to compensate for the loss, which increases power consumption and complexity. ADC manufacturers have responded by integrating the ADC driver 38 with the ADC 40 in a single integrated circuit 36 (IC or “chip”).
Several approaches attempt to reduce the power consumption from the fifty-ohm references. In FIG. 1C, the RF receiver and the anti-alias filter 54 are integrated into a single IC 52. The ADC driver 58 and the ADC 60 are also integrated into a single IC 56. A single fifty-ohm reference 62 is required between the two ICs 52 and 56. An alternative arrangement shown in FIG. 1D integrates the anti-alias filter 74, ADC driver 76 and the RF receiver in a single IC 72. The IC 72 then drivers the ADC 78 through a fifty-ohm reference 80.
A typical measure of nonlinearity used in RF systems is the third order intercept (OIP3, IP3 or TOI). The OIP3 is used to measure the effect of third order products in the bandwidth of interest, typically including the fundamental frequency (tone). FIG. 2 further illustrates the relevance of the third order products. For example, the fundamental f1 102 and f2 104 create second order products 106 and 108 from the sum and difference of the f1 102 and f2 104 frequencies. Similarly, third order products 110 and 112 are created from the sum and difference of the second order harmonic which occurs at twice the frequency of f1 102 and f2 104. The difference signal (2f2−f1) is spectrally close to the fundamental f2 thus it is used in the computation of OIP3 to represent the nonlinearity of the system. A large OIP3 represents a more linear system.
With reference to FIG. 1A, if the desired signal level at the input of the ADC 18 for a maximum signal-to-noise ration (SNR) is 0 dBm, the nonlinearity of the anti-alias filter 16, ADC driver 14 and RF receiver 12 must be higher than 45 dBm. Table 1 compares typical results of anti-alias filters with a 3 dB bandwidth of at least 1 MHz, implemented in a silicon IC. Although the filters listed in Table 1 are not integrated with an RF receiver and an ADC they can be integrated because they are implemented in a CMOS technology. The linearity of the three systems as measured by their respective OIP3 (e.g. 18.5 dBm, 19.5 dBm and 24 dBm) each falls short of the required level of 45 dBm.
TABLE 1Typical anti-alias filter performance3 dB cut-Stop-InputAnti-aliasOIP3offbandreferredfilterIn-bandfrequencyGainrejectionnoisePowerTechnology#119.5 dBm1.92 MHz8.5 dB63 dBIntegrated11.6 mW0.8 μm46 uVrmsBiCMOS#2  24 dBm1, 2.2 or−6 −>Not5 nV/√Hz  55 mW0.13 μm 11 MHz68 dBreportedCMOS#318.5 dBm  15 MHz  0 dB60 dB15.2 nV/√Hz184.8 mW0.6 μm(Low passor 50 nV/√Hzor 17.8 mWCMOSportion)for low power